Augmented transition networks
Woods described a grammar as a network of states with recursive calls, conditions on the arcs and registers for rebuilding structure.
Why it matters
Natural language parsing got a working formalism powerful enough for real queries and simple enough to run.
An ATN adds recursion and memory to a finite-state machine, which is what the models Chomsky criticised in 1956 lacked. The LUNAR system, which answered geologists' questions about lunar samples, was built on it. ATNs remained the standard for parsing until statistical parsers arrived.